Two-headed sharks sound like a monster ripped straight out of a B-list horror movie, but scientists are increasingly finding more of them worldwide.

Some have suggested that the surge in mutants is due to genetic abnormalities triggered by over-fishing.

The puzzling trend started in 2008, when fisherman Christian Johnson caught a two-headed blue shark embryo off the coast of Australia.

In 2013, a group of Floridian fishermen strained to haul in a large Bull shark, but upon gutting it found that its uterus housed a two-headed fetus.

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Blue sharks have so far produced the most two-headed offspring, because they carry large litters of up to 50 babies at a time in the womb.

More recently, Spanish researchers have now found a two-headed Atlantic sawtail catshark embryo while rearing hundreds of sharks for human-health research.

An eagle-eyed scientist spotted it through one of the sharks' characteristic see-through eggs.

The resulting study has been published in the Journal of Fish Biology.

The catfish embryo was no ordinary mutant.

It is the first discovered example of a two-headed shark born by a oviparous shark species - a shark that lays eggs.

The researchers carefully opened the egg in order to study the strange embryo.

Study leader Professor Valentín Sans-Coma is unsure if the embryo would have survived had they left it to hatch naturally.

It is likely that these embryos don't live for long after hatching, which could explain why two-headed egg-laying sharks have never been found before.

What prompted this rising trend in two-headed shark discoveries currently remains a mystery to science.

While their numbers are rising, sightings are few and far between, making it difficult for researchers to pin down exactly what triggers the mythical mutation.

Professor Sans-Coma's team suggest that genetic mutations may be behind their catfish finding.

Their embryos are grown in a lab with almost 800 other specimens, meaning they were unlikely to have exposure to any mutating infections, chemicals or radiation.

Wild sharks' rising mutation rates could come from a variety of factors, including viral infections or pollution.

Some researchers have suggested that over-fishing may be the culprit.

As shark population numbers dwindle, their gene pool shrinks, giving rise to more inbreeding which carries a high risk of passing on crippling genetic abnormalities.

Marine scientist Nicolas Ehemann recently discovered the first two-headed shark ever found in the Caribbean Sea.

Ehemann speculates that the high prevalence of two-headed sharks in nature points to over-fishing as the likely origin.

A master's student at the National Polytechnic Institute in Mexico, Ehemann agrees that the shrinking shark gene pool brought about by fishing will likely lead to rising numbers of birth defects.

Another marine scientist, Dr Felipe Galván-Magaña, believes that the hysteria around two-headed sharks is misplaced.

He argues that the numbers of these sharks aren't growing at all.

In fact, the surge in sightings results from the rising number of new scientific journals to publish in.

Dr Galván-Magaña is no stranger to mutant sharks.

A "cyclops shark," with a single, functioning eye, was caught off the coast of Mexico in 2011 and brought to his lab.

The defect was caused by a congenital condition called cyclopia, which can affect several animal species, including people.

Accessible specimens of mutated sharks are few and far between, making them challenging for scientists to study.

"I would like to study these things, but it's not like you throw out a net and you catch two-headed sharks every so often," says Ehemann.

"It's random."